In the processing of integrated circuits, electric contact must be made to isolated active device regions formed within a wafer/substrate. The active device regions are connected by high electrically conductive paths or lines which are fabricated above an insulator material, which covers the substrate surface. To provide electrical connection between the conductive path and active-device regions, an opening in the insulator is provided to enable the conductive films to contact the desired regions. Such openings are typically referred to as contact openings, or simply "contacts".
As transistor active area dimensions approached one micron in diameter, conventional process parameters resulted in intolerable increased resistance between the active region or area and the conductive layer. The principal way of reducing such contact resistance is by formation of a metal silicide atop the active area prior to application of the conductive film for formation of the conductor runner. One common metal silicide material formed is TiSi.sub.x, where x is predominantly "2". The TiSi.sub.x material is typically provided by first applying a thin layer of titanium atop the wafer which contacts the active areas within the contact openings. Thereafter, the wafer is subjected to a high temperature anneal. This causes the titanium to react with the silicon of the active area, thus forming the TiSi.sub.x. Such a process is said to be self-aligning, as the TiSi.sub.x is only formed where the titanium metal contacts the silicon active regions. The applied titanium film everywhere else overlies an insulative, and substantially non-reactive SiO.sub.2 layer.
Ultimately, an electrically conductive contact filling material such as tungsten would be provided for making electrical connection to the contact. However, tungsten adheres poorly to TiSi.sub.x. Additionally, it is desirable to prevent intermixing of the contact filling material with the silicide and underlying silicon. Accordingly, an intervening layer is typically provided to prevent the diffusion of the silicon and silicide with the plug filling metal, and to effectively adhere the plug filling metal to the underlying substrate. Such material is, accordingly, also electrically conductive and commonly referred to as a "barrier layer" due to the anti-diffusion properties.
One material of choice for use as a glue/diffusion barrier layer is titanium nitride. TiN is an attractive material as a contact diffusion barrier in silicon integrated circuits because it behaves as an impermeable barrier to silicon, and because the activation energy for the diffusion of other impurities is very high. TiN is also chemically thermodynamically very stable, and it exhibits typical low electrical resistivities of the transition metal carbides, borides, or nitrides.
TiN can be provided or formed on the substrate in one of the following manners: a) by evaporating Ti in an N.sub.2 ambient; b) reactively sputtering Ti in an Ar and N.sub.2 mixture; c) sputtering from a TiN target in an inert (Ar) ambient; d) sputter depositing Ti in an Ar ambient and converting it to TiN in a separate plasma nitridation step; or e) by low pressure chemical vapor deposition.
As device dimensions continue to shrink, adequate step coverage within the contact has become problematical with respect to certain deposition techniques. Chemical vapor destination is known to deposit highly conformal layers, and would be preferable for this reason in depositing into deep, narrow contacts. One example prior art technique for depositing TiN is by a low pressure chemical vapor deposition at pressures of less than 1 Torr. Specifically, an example is the reaction of a titanium organometallic precursor of the formula Ti(N(CH.sub.3).sub.2).sub.4, commonly referred to as TMAT, and ammonia in the presence of a carrier gas according to the following formula: EQU Ti(NR.sub.2).sub.4 +NH.sub.3 .fwdarw.TiN+organic byproduct
It is typically desirable in low pressure chemical vapor deposition processes to operate at as low a pressure as possible to assure complete evacuation of potentially undesirable reactive and contaminating components from the chamber. Even small amounts of these materials can result in a significant undesired increase in resistivity. For example, oxygen incorporation into the film before and after deposition results in higher resistivity.
One chemical vapor deposition method has been reported which results in reduced resistivity. Such is described in Katz, "Ohmic Contacts To InP-Based Materials Induced By Means Of Rapid Thermal Low Pressure (Metallorganic) Chemical Vapor Deposition Technique", Journal Of Electronic Materials, Vol. 20, No. 12, pp. 1069-73 (1991) and Katz et al., "The Influence Of Ammonia On Rapid-Thermal Low-Pressure Metalorganic Chemical Vapor Deposited TiN.sub.x Films From Tetrakis (dimethylamido) Titanium Precursor Onto InP", Journal Of Applied Physics, 71(2), pp. 993-1000, Jan. 15, 1992. This process utilizes "rapid thermal" low pressure chemical vapor deposition, in which a wafer is first placed in reactor and the reactor is then sealed. The reactor is then evacuated, and reactive gases are then injected into the reactor which has a cold or unheated environment therein, and an unheated wafer, at this point in the process. Then heat is essentially instantly applied to the wafer to rapidly (within seconds) heat the wafer to the desired process temperature, thus the term "rapid thermal". Such "rapid thermal" processes, however, are not known to have been implemented in a mass production environment at the time of this writing.
It will be noted that the Katz and Katz et al. articles report reduced resistivity at the less conventional process pressures of greater than 5 Torr. Such articles also, however, report substantially constant etch rates despite increasing pressure. Etch rate is generally inversely proportional to density. High density is desired in the resultant TiN film to maximize its diffusion barrier properties. Lower density TiN films do not function as effectively as diffusion barrier materials.
Accordingly, it would be desirable to develop a process for maximizing density of TiN films, while enabling taking advantage of the good conformal deposition provided by CVD techniques.